Solution polymerization processes and their post-reactor finishing designs practice catalyst removal or catalyst deactivation during polymer recovery. The polymer solution from the polymerization is often treated with suitable agents to either neutralize or complex the catalyst residue to aid removal by washing or other means. Drying or devolatilization steps follow with attendant solvent and other material recovery steps. Additives are incorporated via a melt compounding step. These processes are complex and have high costs associated with them.
Non-solution processes, such as gas phase or liquid pool polymerization processes, often use a combination of water or steam treatment followed by a melt mixing step during which the catalyst deactivators and stabilizers are added. Moist nitrogen purging converts the aluminum alkyls to their corresponding alkanes and aluminum oxide, and frees the chlorine from the transition metal chlorides or aluminum alkyl chlorides for removal as hydrogen chloride. However, the transition metal residues will still be present to either form colored complexes with phenolic stabilizers or act as low temperature, oxidation catalysts. With moist nitrogen purging, the magnesium chloride component of, for example, a Ziegler-Natta catalyst system will form a hydrate. On heating above about 120.degree. C. during melt compounding, the hydrated magnesium chloride will also release hydrogen chloride. In the presence of trace amounts of water and acid, the stabilizers will be broken down via dealkylation or ester hydrolysis reactions. These reactions limit the effectiveness of the additives, especially the stabilizers. Such adverse reactions are most likely to occur when the additives are simultaneously introduced into a continuous mixer with olefin polymer from the reactor which has undeactivated catalyst residues and low levels of water.
An improvement with respect to the above is achieved using a melt mixing process with sequential addition of the additives. This process is described in U.S. patent application Ser. No. 625,933, still pending. The process minimizes the chance of any adverse interactions, i.e., between the transition metal catalyst residues and additives such as phenolic or phosphite ester stabilizers. The catalyst residues are first deactivated in a separate step prior to the addition of the stabilizers or other additives. Sequential addition of the catalyst deactivators and the stabilizers in a continuous melt mixer maximizes stabilization efficiency and discoloration resistance (of natural and white pigmented resins) even with trace oxygen present.
Certain transition metal catalyzed olefin polymers having low crystallinity (less than about 10 percent) are inherently sensitive to thermal and photo-oxidation, however. This sensitivity results from high tertiary hydrogen and unsaturation concentrations in combination with catalyst residues or corrosion products, which can act as oxidation catalysts. It would be advantageous, then, to avoid subjecting these heat sensitive polymers to melt compounding for the purpose of catalyst deactivation and incorporation of additives for stabilization and other purposes. Also, the continuous mixer equipment, which is needed for melt compounding, adds significant cost to the polymer finishing process.